Digitally controlled printing is typically accomplished using one of two technologies referred to as “drop-on-demand” and “continuous” inkjet printing. Both printing techniques utilize ink supplies for each color of ink, with the ink being ejected through nozzles formed in a print head.
Drop-on-demand inkjet printing typically uses a thermal or mechanical actuator to provide ink droplets for deposition on a print medium. In continuous ink jet printing technology, ink is typically supplied to an ink reservoir in a print head under pressure so as to produce a jet, or continuous stream of ink from a nozzle in liquid communication with the reservoir. Periodic excitations are imposed on the ink stream to cause the stream to break up into ink droplets.
Some continuous inkjet printers utilize air flow to control the trajectory of ink droplets ejected from a print head, wherein ink droplets can be deflected from their ejection path as they leave the print head to either a print medium or an ink capturing mechanism such as a catcher or gutter. The ink captured by the capturing mechanism can either be recycled back to the ink reservoir for reuse, or disposed of.
Difficulties are often experienced during start-up of continuous stream ink jet printers, when the print head is in an initial dry nozzle plate condition. The ink driving pressure increases from zero but is initially too low to overcome surface tension and drive the ink out of the tiny nozzles in the nozzle plate. A transition period is then reached in which the ink driving pressure overcomes the surface tension effects to force some ink through the nozzles, but the pressure is still insufficient to produce well formed fluid jets of ink. During this transition period from the initial dry nozzle plate condition to fluid jets of ink, ink typically leaks from the print head nozzle and creates a fluid film or beads on the nozzle plate. A similar phenomenon occurs when the printer or print heads are shut down, after which the fluid film or beads can dry on the nozzle plate prior to the next start-up or printing operation of the print head.
A fluid film formed at the nozzle plate increases the probability that fluid leaving the nozzle plate will never overcome the surface tension of the film formed at the nozzles. Fluid beads on the nozzle plate can cause nozzles under the beads to produce a continuous flow of ink that adheres to the nozzle plate. In addition, beads formed adjacent to nozzles can cause misdirection in ink ejected from such nozzles, and inconsistencies in droplet size and shape. The most common solution to clogged jets is to flush the nozzle, or plurality of nozzles with a large amount of ink, however such a method wastes the ink and is not always effective. In addition, this method may not remove the fluid beads from locations adjacent the nozzles, thus misdirected and misshapen drops continue to be ejected from the print head and produce poor quality print images.